Photodiode module and apparatus including multiple photodiode modules

ABSTRACT

Various embodiments of the present invention are directed to a photodiode module including a structure configured to selectively couple light to a dielectric-surface mode of a photonic crystal of the photodiode module. In one embodiment of the present invention, a photodiode module includes a semiconductor structure having a p-region and an n-region. The photodiode module further includes a photonic crystal having a surface positioned adjacent to the semiconductor structure. A diffraction grating of the photodiode module may be positioned and configured to selectively couple light incident on the diffraction grating to a dielectric-surface mode associated with the surface of the photonic crystal. In another embodiment of the present invention, a photodiode apparatus includes multiple, stacked photodiode modules, each of which is configured to selectively absorb light at a selected wavelength or range of wavelengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of application Ser.No. 11/654046, filed on Jan. 17, 2007, which in turn is acontinuation-in-part application of application Ser. No. 11/580,647filed on Oct. 31, 2006, the disclosures of which are incorporated hereinby reference.

TECHNICAL FIELD

The present invention generally relates to photodiodes and applicationsof such photodiodes as photodetectors and solar cells.

BACKGROUND

Photodiodes are used in a variety of applications for converting lightinto electrical signals. For example, photodiodes are employed inphoto-detection applications, such as photodetectors for detecting lightand solar cells for converting solar radiation into electrical energy.FIG. 1 shows one currently available design for a photodiode 10. Thephotodiode 10 includes a p-region 12 made from a p-type semiconductormaterial and an n-region 14 made from an n-type semiconductor materialthat, together, form a p-n junction 16. A depletion region 18 is formedin the p-region 12 and the n-region 14 by majority-carrier holes in thep-region 12 diffusing into the n-region 14 and majority-carrierelectrons in the n-region 14 diffusing into the p-region 12. Thediffusion of majority carriers proceeds until an equilibrium junctionpotential is formed across the depletion region 18 that prevents furtherdiffusion of the majority carriers across the p-n junction 16 fromeither the p-region 12 or the n-region 14.

The junction potential of the depletion region 18 provides the p-njunction 16 with the familiar, nonlinear-current-voltage characteristicsshown in FIG. 2 as I-V curve 20. Under a forward bias voltage (i.e.,positive voltage), the junction potential and the width of the depletionregion 18 is reduced. Majority-carrier electrons from the n-region 14and majority-carrier holes from the p-region 12 have sufficient energyto overcome the junction potential and diffuse across the p-n junction16 to generate a diffusion current. Under a reverse-bias voltage (i.e.,negative voltage within the third quadrant of the graph shown in FIG.2), the junction potential and the width of the depletion region 18increases dramatically, preventing diffusion of majority carriers fromeither the p-region 12 or the n-region 14. However, a supply of minoritycarriers on each side of the p-n junction 16 is generated by thermalexcitation of electron-hole pairs. For example, thermally generatedelectron-hole pairs generated near or in the depletion region 18 on thep-region 12 side of the p-n junction 16 provide minority-carrierelectrons. When the electron-hole pairs are generated within a diffusionlength of the depletion region 18, the minority-carrier electrons candiffuse into the depletion region 18 and the junction potential sweepsthe minority-carrier electrons across the p-n junction 16. Similarly,thermally generated electron-hole pairs generated near or in thedepletion region 18 on the n-region 14 side of the p-n junction 16provide minority-carrier holes and the junction potential sweeps theminority-carrier holes across the p-n junction 16. The drift of minoritycarriers generates a drift or generation current that is relativelyindependent of the applied voltage because the minority carriers aregenerated by an external source, such as thermal or optical energy.

The photodiode 10 may be used as a photodetector for detecting lightincident on the p-n junction 16 by exploiting the voltage independenceof the generation current. As shown in FIG. 1, electron-hole pairs aregenerated by illuminating the p-n junction 16 and surrounding regionswith light having an energy E_(light) greater than the energy-band gapE_(gap) of the semiconductor material used for the p-region 14 and then-region 16. Under a reverse-bias voltage, optically-generated-minoritycarriers within the depletion region 18 or within a diffusion length ofthe depletion region 18 are swept across the p-n junction 16 by thejunction potential to generate an optical-generation current g_(n). Asshown in FIG. 2, the greater the amount of light incident on the p-njunction 16, the greater the magnitude of the optical-generationcurrent. For example, the magnitude of the optical-generation current g₂is greater than the magnitude of the optical-generation current g₁ as aresult of a higher intensity of light illuminating the p-n junction 16and the surrounding regions. Thus, the optical-generation currentgenerated by illuminating the p-n junction 16 with light may be used formeasuring illumination levels.

An array of the photodiodes 10 may also be used to form a solar-cellarray. In a solar-cell array, each of the photodiodes 10 can be operatedin the fourth quadrant of the I-V curves shown in FIG. 2 and connectedto a common bus that delivers power to a load.

In order to maximize the photoconductive response of the photodiode 10,it is important that optically-generated carriers have a sufficientlylong diffusion length so that the optically-generated carriers do notrecombine or become trapped prior to diffusing into the depletion region18 or while being swept across the p-n junction 16. Control of thecarrier diffusion length imposes challenges on designers andmanufacturers of photodiodes that can necessitate using high-quality andhigh-cost semiconductor materials, such as single-crystal semiconductormaterials. It is often desirable for the depletion region 18 to besufficiently wide so that a large fraction of the intensity of light isabsorbed within the depletion region 18. While increasing the width ofthe depletion region 18 can improve the optical efficiency of thephotodiode 10, it can also deleteriously decrease the response time ofthe photodiode 10. Therefore, manufacturers and designers of photodiodescontinue to seek improved photodiodes in which cheaper, lower-qualitysemiconductor materials can be utilized without substantially degradingphotodiode performance.

SUMMARY

Various embodiments of the present invention are directed to aphotodiode module including a structure configured to selectively couplelight to a dielectric-surface mode of a photonic crystal of thephotodiode module. In one embodiment of the present invention, aphotodiode module includes a semiconductor structure having a p-regionand an n-region. The photodiode module further includes a photoniccrystal having a surface positioned adjacent to the semiconductorstructure. A diffraction grating of the photodiode module may bepositioned and configured to selectively couple light incident on thediffraction grating to a dielectric-surface mode associated with thesurface of the photonic crystal. In another embodiment of the presentinvention, a photodiode apparatus includes multiple, stacked photodiodemodules, each of which is configured to selectively absorb light at aselected wavelength or range of wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate various embodiments of the present invention,wherein like reference numerals refer to like or similar elements indifferent views or embodiments shown in the drawings.

FIG. 1 is a schematic illustration of a currently available photodiodedesign.

FIG. 2 is a graph depicting the current-voltage characteristics of thephotodiode shown in FIG. 1 with and without optical stimulation.

FIG. 3 is a schematic isometric view of a photodiode module including ap-n junction according to one embodiment of the present invention.

FIG. 4A is a schematic cross-sectional view taken along line A-A shownin FIG. 3 showing one embodiment of the present invention for adiffraction grating configuration and further showing an intensityprofile of a dielectric-surface mode absorbed within the p-nsemiconductor structure of the photodiode module.

FIG. 4B is a cross-sectional view taken along line A-A shown in FIG. 3showing another embodiment of the present invention for a diffractiongrating configuration.

FIG. 5 is a schematic, exploded, isometric view of the photodiode moduleshown in FIG. 3.

FIG. 6 is a schematic cross-sectional view of a photodiode moduleincluding a p-i-n semiconductor structure according to anotherembodiment of the present invention.

FIG. 7 is an isometric view of a photodiode apparatus including multiplephotodiode modules according to yet another embodiment of the presentinvention.

FIG. 8 is a schematic, exploded, cross-sectional view of the photodiodeapparatus shown in FIG. 7 showing an intensity profile of individualdielectric-surface modes absorbed by individual p-n junctions of firstand second photodiode modules and an intensity profile of asurface-plasmon mode absorbed within a semiconductor material of a thirdphotodiode module.

FIG. 9 is a plan view of an array of photodiode modules/apparatusesaccording to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE VARIOUS EMBODIMENTS

Various embodiments of the present invention are directed to aphotodiode module including a structure configured to selectively couplelight to a dielectric-surface mode of a photonic crystal of thephotodiode module, and a photodiode apparatus that may employ a numberof such photodiode modules. The disclosed photodiode modules andapparatuses may be utilized as photodetectors, solar cells, and a numberof other applications.

FIG. 3 shows a photodiode module 30 according to one embodiment of thepresent invention. The photodiode module 30 includes a p-n semiconductorstructure 32 having a p-region 34 made from a p-type semiconductormaterial, an n-region 36 made from an n-type semiconductor material, anda p-n junction 38 formed between the p-region 34 and the n-region 36.The semiconductor materials may be formed from a variety of singlecrystal, polycrystalline, and amorphous semiconductor materials that areselected for a particular optical application. A depletion region 33 isformed within a portion of the p-region 34 and a portion of the n-region36 due to majority-carrier holes in the p-region 32 diffusing into then-region 36 and majority-carrier electrons in the n-region 36 diffusinginto the p-region 34. The diffusion of majority carriers proceeds untilan equilibrium junction potential is formed across the depletion region33 that prevents further diffusion of the majority carriers across thep-n junction 38 from either the p-region 34 or the n-region 36.

A number of recesses 42 arranged in a selected periodic pattern may beformed in the p-n semiconductor structure 32 to define a diffractiongrating 40. For example, the recesses 42 may be formed with a spacing din a two-dimensional pattern as shown in the embodiment of FIG. 3, aone-dimensional pattern, or another suitable pattern. As will bediscussed in more detail below, the diffraction grating 40 is configuredto selectively couple electromagnetic radiation of a selected wavelengthor range of wavelengths incident on the diffraction grating 40 to adielectric-surface mode of a photonic crystal 44 positioned adjacent tothe p-n semiconductor structure 32. According to various embodiments ofthe present invention, the recesses 42 of the diffraction grating 40 mayextended within only the p-region 34 as shown in FIG. 4A or the recesses42 may extended within both the p-region 34 and the n-region 36 as shownin FIG. 4B. In another embodiment of the present invention, a separatedielectric layer having a diffraction grating similarly structured asthe diffraction grating 40 may be positioned adjacent to the p-region 34of the p-n semiconductor structure 32.

Referring again to FIG. 3, the photonic crystal 44 of the photodiodemodule 30 has an interfacial surface 46 that is positioned adjacent tothe n-region 36 of the p-n semiconductor structure 32. In theillustrated embodiment shown in FIG. 3, the photonic crystal 44 is aone-dimensional photonic crystal. Shaded dielectric layers 48 and 49 arecomprised of a first dielectric material having a dielectric constant,and un-shaded dielectric layers 50 and 51 are comprised of a seconddielectric material having a different dielectric constant. Thedielectric layers 48-51 are periodically spaced with a repeat distancea, and the periodic arrangement of the dielectric layers 48-51 resultsin a photonic-band gap in which one or more range offrequencies/wavelengths of electromagnetic radiation is prevented frompropagating in a direction generally perpendicular to the dielectriclayers 48-51 of the photonic crystal 44. The dielectric layers 48-49 mayhave approximately the same dielectric constant as the p-region 34 andthe dielectric layers 50-51 may have approximately the same dielectricconstant as the n-region 36. As merely an example, the thicknesses ofeach of the dielectric layers 48-51 may about 100 nm to about 300 nm. Aswill be discussed in more detail below, the photonic-band gap of thephotonic crystal 44 is designed to allow transmission of light of aselected frequency/wavelength range that is not coupled to thedielectric-surface mode of the photonic crystal 44.

It should be noted that the photonic crystal 44 may include moredielectric layers than the four shown in FIG. 3, and the number ofdielectric layers shown in FIG. 3 is merely for illustrative purposes.Moreover, the photonic crystal 44 does not need to be a one-dimensionalphotonic crystal. In other embodiments of the present invention, thephotonic crystal 44 may be a two- or three-dimensional photonic crystalexhibiting a photonic-band gap in more than one direction. Accordinglythe term “photonic crystal,” as used herein includes one-dimensionalphotonic crystals (e.g., a periodic stack comprising layers of twodifferent dielectrics that alternate periodically in one direction),two-dimensional photonic crystals, and three-dimensional photoniccrystals.

The operation of the photodiode module 30 as a photodetector is bestunderstood with reference to FIG. 5. Free-space light 52 having a rangeof wavelengths λ₁, λ₂ . . . λ_(n) is incident on the diffraction grating40 of the p-n semiconductor structure 32. The diffraction grating 40 isstructured to selectively couple light having a wavelength λ₁ to adielectric-surface mode 54 (FIG. 4A) associated with the interfacialsurface 46 of the photonic crystal 44. The un-coupled light at thewavelengths λ₂ . . . λ_(n) that are not coupled to thedielectric-surface mode 54 is transmitted through the p-n semiconductorstructure 32 because the un-coupled light at the wavelengths λ₂ . . .λ_(n) has an energy less than the energy-band gap of the semiconductormaterials used for the p-n semiconductor structure 32. The un-coupledlight at wavelengths λ₂ . . . λ_(n) also propagates through the photoniccrystal 44 because the light at the wavelengths λ₂ . . . λ_(n) fallsoutside the forbidden wavelengths of the photonic crystal 44'sphotonic-band gap.

Turning again to FIG. 4A, as previously described, the diffractiongrating 40 couples light of a selected wavelength or range ofwavelengths, such as the light at the wavelength λ₁, to thedielectric-surface mode 54 of the photonic crystal 44. The intensity ofthe dielectric-surface mode 54 may be about one to about three orders ofmagnitude greater than the intensity of the incident free-space light atthe wavelength λ₁. Thus, the diffraction grating 40 enhances theintensity of the incident light at the wavelength λ₁ received by thephotodiode module 30. At a surface of a photonic crystal, adielectric-surface mode exists in which an electromagnetic wave may begenerally confined to the surface and regions immediately adjacent tothe surface. The resonance condition that permits transfer of energyfrom the light at the wavelength λ₁ to the dielectric-surface mode 54generally requires that both the energy and momentum of the light at thewavelength λ₁ match the energy and momentum of the dielectric-surfacemode 54. Irradiating the diffraction grating 40 with the free-spacelight 52 causes the light at the wavelength λ₁ to diffract and thediffracted light having a wavevector equal to and aligned with thewavevector of the dielectric-surface mode 54 (i.e., in a directiongenerally parallel to the x-axis direction) is coupled to thedielectric-surface mode 54 of the photonic crystal 44.

FIG. 4A shows the intensity distribution of the dielectric-surface mode54 propagating along the interfacial surface 46 generally in an x-axisdirection. The intensity decays exponentially in a direction toward thep-n semiconductor structure 32 and also decays exponentially in adirection into the bulk of the photonic crystal 44. A large fraction ofthe intensity of the dielectric-surface mode 54 is absorbed by the p-nsemiconductor structure 32. Depending upon the number of dielectriclayers that comprises the photonic crystal 44, the maximum intensity ofthe dielectric-surface mode 54 may be positioned within the depletionregion 33 of the p-n semiconductor structure 32 by proper design of thephotonic crystal 44 to increase the number of electron-hole pairsgenerated within the depletion region 33. Electron-hole pairs aregenerated in the p-n semiconductor structure 32 responsive to absorptionof the dielectric-surface mode 54 in the p-n semiconductor structure 32.When the p-n junction 38 of the p-n semiconductor structure 32 isreverse biased using a voltage source 45, a current is generated acrossthe p-n junction 38 that is generally independent of the applied voltageand proportional to the amount of light at the wavelength λ₁ coupled tothe dielectric-surface mode 54.

In a mode of operation according to another embodiment of the presentinvention suitable for utilizing the photodiode module 30 as one of manysolar cells in a solar-cell array, the photodiode module 30 may beforward biased to the fourth quadrant of the I-V curve (FIG. 2) wherethe voltage is positive and the current generated is negative. In suchan embodiment of the present invention, the diffraction grating 40 maybe configured to couple light having a wavelength between about 575 nmto about 585 nm, which is about the peak wavelength of the solarspectrum (i.e., yellow), to the dielectric-surface mode 46 of thephotonic crystal 44.

Due to the high-intensity of the dielectric-surface mode 54, the p-nsemiconductor structure 32 may be fabricated with a thickness of about10 nm to about 30 nm, while still absorbing the same or greater amountof light and generating the same or greater number of electron-holepairs than that of a p-n semiconductor structure made from the same orsimilar material with a thickness of, for example 1 μm. As such, the p-nsemiconductor structure 32 may be fabricated from relatively low-qualitysemiconductor materials, such as polycrystalline silicon, amorphoussilicon, or single-crystal semiconductor materials that may have asignificant amount of crystalline or other defects. Because the p-nsemiconductor structure 32, depletion region 33, or both may have athickness that is less than the diffusion length of electrons and holesgenerated by the intense dielectric-surface mode 54, the electron andholes do not recombine or become trapped by defect sites within the p-nsemiconductor structure 32 prior to being swept across the p-n junction38. Moreover, due to the depletion region 33 of the p-n semiconductorstructure 32 being relatively thin (e.g., a thickness of about 10 nm toabout 30 nm), the response time of the photodiode module 30 may beincreased because the minority carriers generated by absorption of thedielectric-surface mode 54 do not have to travel a substantial distanceto reach the depletion region 33 prior to being swept across the p-njunction 38. Therefore, the detection capability of the photodiodemodule 30 may not be degraded while allowing for cheaper and easierfabrication of the p-n semiconductor structure 32 with less pure, higherdefect semiconductor materials.

As can be appreciated from FIGS. 3 through 5, the photodiode module 30may be used for detecting light of a selected wavelength (e.g., λ₁ inFIG. 5), while also allowing other, un-detected, wavelengths of light(e.g., λ₂ in FIG. 5) to be transmitted through the photodiode module 30substantially un-attenuated. Accordingly, the photodiode module 30 maybe used in optical-fiber-communication applications that requireselectively detecting certain wavelengths of light propagating in anoptical fiber, while allow signals having other wavelengths not absorbedby the photodiode module 30 to be transmitted through the photodiodemodule 30 for further processing or use.

In other embodiments of the present invention, a p-i-n semiconductorstructure may be used instead of a p-n junction. FIG. 6 shows aphotodiode module 60 according to another embodiment of the presentinvention. The photodiode module 60 includes a p-i-n semiconductorstructure 62 having a p-region 64, an n-region 66, and an intrinsicregion 68 located between the p-region 64 and the n-region 66. Theintrinsic region 68 may be made from an intrinsic semiconductor materialor a semiconductor material having a resistance that is substantiallygreater than that of the p-region 64 and the n-region 66. A depletionregion is formed across the intrinsic region 68 and, unlike thedepletion region 38 of the p-n semiconductor structure 32 shown in FIG.3, the width of the depletion region is relatively independent of anapplied voltage. As with the photodiode module 30, a diffraction grating70 including a number of recesses 72 arranged in a selected periodicpattern may be formed in the p-region 64, as shown in FIG. 6, or therecesses 72 may extend into the intrinsic region 68 and/or the n-region66. The photodiode module 60 further includes a photonic crystal 74having an interfacial surface 75 capable of supporting adielectric-surface mode 85, which may be a one-dimensional photoniccrystal. The photonic crystal 74 may be comprised of a periodicarrangement of dielectric layers 76-84 that generally reproduces thedielectric-constant profile of the p-i-n semiconductor structure 62. Thedielectric layers 76-78 may have a dielectric constant that isapproximately the same as the dielectric constant of the p-region 64,the dielectric layers 79-81 may have a dielectric constant that isapproximately the same as the dielectric constant of the n-region 66,and the dielectric layers 82-84 may have a dielectric constant that isapproximately the same as the dielectric constant of the intrinsicregion 68.

The photodiode module 60 functions similarly to the photodiode module 30shown in FIG. 3. Free-space light 86 having a range of wavelengths λ₁,λ₂ . . . λ_(n) is incident on the diffraction grating 70. The light atthe wavelength λ₁ is selectively coupled to the dielectric-surface mode85 of the interfacial surface 75 of the photonic crystal 74. As shown inFIG. 6, the photonic crystal 74 may be designed so that maximumintensity of the dielectric-surface mode 85 is positioned within theintrinsic region 68 of the p-i-n semiconductor structure 62. As a resultof the intense dielectric-surface mode 85 extending within the p-i-nsemiconductor structure 62, electron-hole pairs are generated within thep-i-n semiconductor structure 62 and, consequently, a currentproportional to the amount of light at the wavelength λ₁. The lighthaving the wavelengths 2 ₂ . . . λ_(n) that is not coupled to thedielectric-surface mode 85 is transmitted through the semiconductorstructure 62 and the photonic crystal 74.

FIGS. 7 and 8 show a photodiode apparatus 100 including a number ofstacked photodiode modules 102-104, each of which is configured to beselectively absorb a different wavelength or range of wavelengths oflight. Each of the photodiode modules 102 and 103 may be configured asthe photodiode modules 30 or 60 shown in FIGS. 3 and 6, respectively.The photodiode module 102 includes a p-n semiconductor structure 106with a diffraction grating 105 and a photonic crystal 108 with aninterfacial surface 107 positioned adjacent to the p-n semiconductorstructure 106. The diffraction grating 105 is configured to selectivelycouple light having a wavelength λ₁ to a dielectric-surface modesupported at the interfacial surface 107. The photodiode module 103includes a p-n semiconductor structure 110 with a diffraction grating109 and a photonic crystal 112 with an interfacial surface 113positioned adjacent to the p-n semiconductor structure 110. Thediffraction grating 109 is configured to selectively couple light havinga wavelength λ₂ to a dielectric-surface mode supported at an interfacialsurface 113. Although the photodiode modules 102 and 103 are illustratedusing p-n semiconductor structures with respective p-n junctions, thephotodiode modules 102 and 103 may utilize p-i-n semiconductorstructures as previously described with respect to the photodiode module60 shown in FIG. 6.

The photodiode module 104 of photodiode apparatus 100 may be configuredas a metal-semiconductor-metal (“M-S-M”) photodiode. A MSM photodiodemay be utilized to reduce the size of the photodiode module 104 relativeto the size of the photodiode modules 102 and 103. However, in otherembodiments of the present invention, a photodiode module similar to thephotodiode modules 30 and 60 shown in FIGS. 3 and 6, respectively, maybe used to selectively allow certain wavelengths of light to betransmitted through the photodiode apparatus 100. The photodiode module104 includes a metallic electrode 116 including a diffraction grating117 similar to the diffraction gratings previously discussed above, anelectrode 118, and a semiconductor material 114 positioned between theelectrodes 116 and 118 and in electrical contact with electrodes 116 and118. The metallic electrode 116 may comprise gold, silver, platinum,copper, aluminum, chromium, alloys of any of the preceding metals, oranother suitable material. The diffraction grating 117 is configured toselectively couple light at a wavelength λ₃ to a surface-plasmon modesupported by the metallic electrode 116.

As shown in FIG. 8, in operation, free-space light 120 including a rangeof wavelengths λ₁, λ₂, λ₃ . . . λ_(n) is incident on the diffractiongrating 105 of the photodiode module 102. The diffraction grating 105couples light at the wavelength λ₁ to a dielectric-surface mode 122 thatpropagates along the interfacial surface 107 and enhances the intensityof the light at the wavelength λ₁. The light at the wavelengths λ₂ andλ₃ is transmitted through the p-n semiconductor structure 106 and thephotonic crystal 108 because the light at wavelengths λ₂ and λ₃ fallswithin the energy-band gap of the p-n semiconductor structure 106 anddoes not fall within the forbidden range of wavelengths of thephotonic-band gap of the photonic crystal 108. The light at thewavelengths λ₂ and λ₃ is transmitted through the photodiode module 102and is incident on the diffraction grating 109 of the photodiode module103. The diffraction grating 109 selectively couples light at thewavelength λ₂ to a dielectric-surface mode 124 that propagates along theinterfacial surface 113 of the photonic crystal 112. The coupling of thelight at the wavelength λ₂ to the dielectric-surface mode 124 enhancesthe intensity of the light at the wavelength λ₂. The light at thewavelength λ₃ is transmitted through the p-n semiconductor structure 110and the photonic crystal 112 because the light at the wavelength λ₃falls within the energy-band gap of the p-n semiconductor structure 110and does not fall within the forbidden range of wavelengths of thephotonic-band gap of the photonic crystal 112. The light at thewavelength λ₃ is transmitted through the photodiode module 103 and isincident on the diffraction grating 117 of the photodiode module 104.The diffraction grating 117 selectively couples light at the wavelengthλ₃ to a surface-plasmon mode 126 that propagates along the interfacialsurface 121 between the semiconductor material 114 and the metallicelectrode 116. A surface-plasmon mode is an electromagnetic fieldgenerally confined to an interface between a metallic material having anegative dielectric constant and another medium having a positivedielectric constant. The surface-plasmon mode is a result of thecollective oscillations of the free-electron gas of the metallicmaterial. The coupling of the light at the wavelength λ₃ to thesurface-plasmon mode 126 enhances the intensity of the light at thewavelength λ₃.

In a mode of operation according to an embodiment of the presentinvention, the photodiode modules 102-104 may be employed asphotodetectors. In such a mode of operation, each of the photodiodemodules 102-104 may be individually reversed biased, and thephoto-current generated in each of the photodiode modules 102-104responsive to the absorption of the light at the wavelengths λ₁, λ₂, andλ₃, respectively, may be used for determining the amount of lightabsorbed by the photodiode modules 102, 103, and 104 at the wavelengthsλ₁, λ₂, and λ₃, respectively in a manner similar to the photodiodemodule 30 shown and described with respect to FIG. 3.

In another embodiment of the present invention suitable for utilizingthe photodiode apparatus 100 as the individual solar cells of asolar-cell array, the photodiode apparatus 100 may be forward biased tothe fourth quadrant of the I-V curve (FIG. 2) where the voltage ispositive and the current generated is negative. When the photodiodeapparatus 100 is used as a solar cell, the multiple photodiode modules102-104 enables increasing the quantum efficiency of the photodiodeapparatus 100 because each of the photodiode modules 102-104 may beoptimized to absorb solar radiation at one of the peak wavelengths ofthe solar spectrum. For example, FIG. 9 shows yet another embodiment ofthe present invention in which any of the preceding embodiments ofphotodiodes modules and photodiode apparatuses may be arranged as anarray 130 including a number of photodiode modules or apparatuses132-151. Each of the photodiode modules or apparatuses 132-151 may beconnected to a common bus (not shown) and the solar generated electricalcurrent generated by each of the photodiode modules or apparatuses132-151 may be delivered the common bus and further to a load.

Although the present invention has been described in terms of particularembodiments, it is not intended that the present invention be limited tothese embodiments. Modifications within the spirit of the presentinvention will be apparent to those skilled in the art. For example, inanother embodiment of the present invention, one or more dielectriclayers may be interposed between the p-n or p-i-n semiconductorstructure and the photonic crystal. The diffraction gratingconfiguration may also depart from the diffraction grating configurationshown in FIGS. 3 through 7. For example, the recesses may have othergeometries than the rectangular-shaped recesses shown in FIGS. 3 through7 such as, circular or another suitable geometry. Furthermore, otherperiodic patterns may be used for the diffraction grating that theillustrated diffraction gratings.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the presentinvention. The foregoing descriptions of specific embodiments of thepresent invention are presented for purposes of illustration anddescription. They are not intended to be exhaustive of or to limit thepresent invention to the precise forms disclosed. Obviously, manymodifications and variations are possible in view of the aboveteachings. The embodiments are shown and described in order to bestexplain the principles of the present invention and its practicalapplications, to thereby enable others skilled in the art to bestutilize the present invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the present invention be defined by theclaims and their equivalents.

1. A method of selectively coupling light to a photodiode apparatus, themethod comprising: irradiating a first diffraction grating of a firstphotodiode module with light to selectively couple at least a portion ofthe light to a first dielectric-surface mode associated with a firstsurface of the first photodiode module; and transmitting lightun-coupled to the first dielectric-surface mode through the firstphotodiode module.
 2. The method of claim 1, further comprisingirradiating a second diffraction grating of a second photodiode modulewith the light un-coupled to the first dielectric-surface mode toselectively couple at least a portion of the light un-coupled to thefirst dielectric-surface mode to a second dielectric-surface modeassociated with a second surface of the second photodiode module.
 3. Themethod of claim 1 wherein the first photodiode module comprises: a firstsemiconductor structure including a p-region and an n-region; a firstphotonic crystal including the first surface positioned adjacent to thefirst semiconductor structure; and the first diffraction grating toreceive the light and to selectively couple the light to the firstdielectric-surface mode.
 4. The method of claim 3 wherein a maximumintensity of the first dielectric-surface mode is located in a depletionregion of the first semiconductor structure.
 5. The method of claim 3wherein the first dielectric-surface mode is to increase electron-holepair generation in the first semiconductor structure.
 6. The method ofclaim 2, further comprising irradiating a third diffraction grating of athird photodiode module with light un-coupled to the seconddielectric-surface mode transmitted through the second photodiode moduleto selectively couple at least a portion of the light un-coupled to thesecond dielectric-surface mode to a surface-plasmon mode associated witha third surface of the third photodiode module.
 7. The method of claim 5wherein the third photodiode module comprises ametal-semiconductor-metal photodiode.
 8. The method of claim 1, furthercomprising: applying a reverse bias to the first photodiode module, anddetecting a change in a photocurrent generated in the first photodiodemodule when the first diffraction grating is irradiated with the light.